I would probably enjoy the D610 if it was not for the grip, the only thing that bugs me about those camera's (haven't had enough time playing with their RAW files + skin tones, but from what I've tried it has bottomless shadow noise to the point that you can not use simple tone curve adjustments any more without throwing things out of whack but need to do layer mask adjustments to really take advantage). There is a pointy bulge on the back of the grip that aggravates me...and if I change my grip to 'avoid' that bulge it doesn't feel very secure to me.

The shadows of the 6D are cleaner than 5DMKII or 5DMKIII. The 5DMKIII was a very minor 'debanding' type of improvement over the 5DMKII (similar to say how much a 60D or recent t5i may do over the old 7D). The 6D actually offers up enough improvement in shadow noise to be mostly acceptable and matches IMO the Nikon D3200. Noticed I said D3200, not D600.

I actually have a tendency to not raise up my ISO enough in many of my shots and shoot a enough lowlight that the 6D is fine for me (it actually pulls a head a little at very high ISO shots).

Other things the 6D does...wifi/gps, ludicrously good center point AF, like a glove ergonomics (for me). The Nikon 50mms are seriously more modern than Canons, although the Canon 50mm 1.4 is still very very good for the price, my copy is quite sharp (the Nikon 50mm 1.8 is arguably better in price perf than the Nikon 50 1.4 which from what I've seen from samples or charts just requires some stop-down to get going). The Canon 50mm seems to be the #1 most likely prime update to come in the next year or so.